INDIANAPOLIS — Health advocates working to enroll uninsured Indiana residents through the federal exchange that opened Tuesday said most people appear to be reviewing the new marketplace at the center of the health care overhaul before actually buying health insurance.

Indiana residents began creating accounts through the federal insurance exchange, also called a marketplace, along with residents of 35 other states where leaders opted to let the U.S. Department of Health and Human Services run their exchanges.

The exchange represents the core of President Barack Obama's sweeping health care overhaul, signed into law more than three years ago, which includes a mandate that residents have health insurance or face tax penalties.

For now, at least, it looks like residents are still trying to understand exactly what the exchange is and what their options are, said Marla Asberry, an outreach specialist with Open Door Health Services and one of the federal navigators assisting residents. Asberry and her staff spent the day fielding calls from residents with questions about the exchange.

"I could see the middle of the enrollment clear up to the end" being busy, she said Tuesday.

Customers must enroll by Dec. 15 for coverage to start Jan. 1. Customers have until the end of March to sign up in order to avoid tax penalties.

Opening day for the federal exchange was filled with extensive delays and technical problems, including missing buttons and password security questions on the online application. Federal officials attributed the slowdown to the surprisingly high volume of interest in the exchange on its first day of operation.

Marilyn Tavenner, administrator for the Centers for Medicare and Medicaid Services, told reporters Tuesday that the federal insurance website had seen 2.8 million hits, but would not say how many of those visitors had actually enrolled in a new plan through the site.

Rates released by HHS last week showed Indiana residents will likely pay between $100 and $1,000 depending on the size of their family, how much they earn and how much they qualify for in federal tax credits. Indiana officials estimate more than 500,000 residents are eligible for subsidized insurance through the federal exchange.

Residents will buy plans from private insurers, but will have to go through the federal exchange in order to receive the tax subsidies.

A spokesman for Indiana's Family and Social Services, which is expecting a jump in Medicaid applications with the start of insurance mandate, said the state saw "very little or no increase at all" in the number of calls it received Tuesday.

Republican Gov. Mike Pence elected to let the federal government run the exchange in Indiana, but the state has added temporary staff to deal with the expected increase in insurance questions and applications.

Heather McCabe, a professor of social work at Indiana University-Purdue University Indianapolis and expert on the health care law, also said a lot of residents are still figuring out exactly what the insurance exchange is and examining their options.

"I think that it is good we are seeing so much interest, even if the interest is to see what is going on with it," she said Tuesday.

The opening of the exchange came the same day as a shutdown of the federal government for the first time in 17 years. While the core of the federal staff continued working on the exchange, regional aides like the specialist overseeing Indiana were furloughed for the day.